1. Field of the Invention
The present invention relates generally to torque indicators and, more particularly, to a mechanical torque indicator positionable between a pair of rotatable members and providing a visual indication of the magnitude of the torque.
2. Discussion of the Prior Art
In the installation of earth anchors, it is known to employ a hydraulic drive motor attached through a suitable installation torque head to an anchor installing tool in order to deliver a rotary driving force to the anchor installing tool. An anchor attached to an elongated rod is received within the anchor installing tool, and the anchor rotates with the tool. The anchor includes a helical load-bearing element which penetrates the earth as the anchor is rotated in order to position the anchor and rod at a desired depth beneath the surface of the ground.
There is a known relationship between the load-bearing capacity of an earth anchor and the torque with which the anchor is installed; specifically that as the torque required to install the anchor increases, the load-bearing capacity of the anchor to carry loads exerted on the rod to which the anchor is attached also increases. Thus, if the magnitude of the potential load to be carried by the anchor is known, then it is possible to install the anchor until the torque required to turn the anchor reaches a magnitude corresponding to the magnitude of the potential load.
In order to monitor the torque between an installation torque head and an anchor installing tool, and thus to gage the torque exerted on the anchor being installed by the tool, it is conventional to position a torque indicator between the torque head and tool. An example of a known type of torque indicator is a shear pin torque indicator which includes a pair of axially aligned cylindrical disks which are movable relative to one another. One of the disks is fastened to the torque head and the other to the installing tool, and one or more shear pins are placed between the two disks in order to prevent the disks from rotating relative to one another until the torque experienced by the disks is sufficient to shear the pins. After shearing of the pins, the indicator becomes "free wheeling" to prevent the anchor from being driven deeper.
By controlling the number of shear pins used in a shear pin torque indicator, it is possible to set the magnitude of torque at which installation of the anchor is considered complete.
Another known type of torque indicator is a hydraulic indicator which includes a hydraulic load cell interposed between a pair of disks such that the torque experienced by the disks causes a change of pressure within the load cell which is represented by a visible gauge mounted on the indicator.
Another known type of torque indicator is illustrated in U.S. Pat. No. 4,808,976, to Kiefer et al., and includes a pair of spaced cylindrical elements and a contact support connected with each of the elements. Each of the supports is provided with a contact that is spaced from the contact on the other support by a predetermined distance while the elements are free of torsional load. When torque is applied to the elements, the contacts move relative to one another, and when a preset magnitude of torque is reached, the contacts close a circuit including an audible alarm.